Integrins on cancer cells play important roles in tumor invasion and spread. They are a family of proteins found on the cell surface of many cell types that mediate interactions between cells, and between cells and their surroundings. Integrins are heterodimers, composed of α and β subunits involved in cell-cell and cell-substratum interactions. Integrins serve as receptors for extracellular matrix proteins such as fibronectin, fibrinogen, vitronectin, collagen and laminin. Some of these interactions have been shown to be mediated via an Arg-Gly-Asp (RGD) sequence present in the matrix proteins. Both the α and β subunits are required for fibrinogen binding. For example, one of the members of the superfamily of integrin cell surface receptors is the platelet membrane glycoprotein (GP)IIb/IIIa which interacts with plasma fibrinogen in platelet aggregation.
CN binds to a specific integrin on the surface of blood platelets, and blocks the ability of platelets to adhere to one another (a process called platelet aggregation). Platelets are small fragments of bone marrow cells that are found in the blood stream. They have both beneficial and harmful activities. Their useful action is to stop bleeding following injury by facilitating the formation of a blood clot. But, under certain conditions they are involved in blocking arteries that supply nourishment to the heart—an action that can lead to a heart attack.
Integrin cell surface receptors have been investigated in the role of platelets in mediating coronary artery thrombosis and rethrombosis in the genesis of acute myocardial infarction [Zucker, M. B., Sci. American 242:86 (1990)]. For platelet aggregation an RGD sequence present in fibrinogen is essential for the interaction with (GP)IIb/IIIa [Ginsberg, M. H. et al., Thrombos. Haemostas. 59:1 (1988)]. Because of its inhibition of platelet aggregation, snake venom has been the subject of various investigations.
A number of proteins purified from venom of snakes of the Crotalidae and Viperidae families have been found to inhibit glycoprotein (GP)IIb/IIIa mediated platelet aggregation [see, e.g., Huang, T. F. et al., J. Biol. Chem. 262:16157 (1987); Gan, Z. R. et al., J. Biol. Chem. 263:19827 (1988); Yasuda, T. et al., J. Am. Coll. Cardiol. 16:714 (1990); Trikha, M. et al., Fibrinolysis 4 (Suppl. 1):105 (1990); Trikha, M. et al., Blood 76 (Suppl. 1):479a (1990); Holahan, M. A. et al., Pharmacology 42:340 (1991); Shebuski, R. J. et al., Circulation 82:169 (1990); Yasuda, T. et al., Circulation 83:1038 (1991)]. These proteins, classified as disintegrins, are typically disulfide rich. Moreover, all disintegrins isolated thus far, with the exception of barbourin [Scarborough, R. M. et al., J. Biol. Chem. 266:9359 (1991)] contain an RGD (Arg-Gly-Asp) sequence that has been implicated as being involved in the inhibition of integrin-mediated interactions. In particular, the RGD sequence of the disintegrins may compete for fibrinogen binding sites on the platelet membrane, thereby inhibiting platelet aggregation induced by ADP or other agents.
Nonetheless, there appears to be increasing evidence that disintegrins may have unique surface geometry which facilitates interactions with integrins by mechanisms other than those based solely upon the RGD site. For example, the finding that a mutated, chemically synthesized derivative of echistatin (in which alanine was substituted for arginine in the RGD sequence) still possessed some biological activity, suggests that other regions in the protein may be involved in binding and that there may be some flexibility in the RGD binding site [Connolly, T. M. et al., Circulation 82 (Suppl. III):660 (1990)]. Synthetic RGD peptides, due to their small size, generally do not possess the molecular topography of the disintegrins and therefore cannot interact via the multiplicity of mechanisms likely to be involved in disintegrin binding.
One disintegrin of particular interest is CN, which has been isolated from the venom of Agkistrodon contortrix contortrix (the southern copperhead snake). The originally-reported purification procedure included molecular sieve chromatography on Sephadex G-100 SF, desalting on Sephadex G-25F and reverse phase HPLC. ADP-enhanced aggregation of stirred human platelet rich plasma and the inhibition thereof by CN were monitored at 37° C. It was found that preincubation for 1 minute of the platelet rich plasma (3×105/mm3) with 5 μl of the low molecular weight peak after Sephadex G-100 SF resulted in 76% inhibition of platelet aggregation induced by 10 μM ADP [Trikha et al. (1990), supra].
In a subsequent report it was noted that in crude venom, the inhibitor was not readily detectable due to the presence of platelet aggregating activity; however, following the first step of purification (hydrophobic interaction HPLC) inhibitory activity was separated from both aggregating activity and an α-chain degrading fibrinolytic enzyme present in the venom. Inhibitory activity was pooled following HPLC and applied to a hydroxylapatite HPLC column. In the final step of purification, C4 reverse phase HPLC chromatography was employed. The yield of the homogeneous protein was 3-5 mg per gram of venom. CN was reported to have a molecular weight of 18-21 kDa under non-reducing conditions and 9 kDa under reducing conditions; thus, the molecule was believed to be a homodimer with the two subunits being held together by disulfide bond(s). Isoelectric focusing showed that the protein had an acidic pI. CN was reported not to exhibit fibrinolytic activity and was not a 5′-nucleotidase or a phospholipase based on molecular size and kinetics of inhibition of platelet aggregation. Following preincubation for 1 minute, CN at approximately 100 nM was reported to completely inhibit ADP-induced platelet aggregation [Trikha et al. (1990), supra].
It has further been reported that CN has 65 amino acids with five to six disulfide bridges, and that the sequence of CN appears to begin 10 amino acids downstream of applaggin (a platelet aggregation inhibitor from the venom of Agkistrodon piscivorus piscivorus). It was speculated that CN may have an insertion and/or a C-terminal extension of nine amino acids. It was further reported that a 50% inhibition (IC50) of human platelet aggregation in platelet rich plasma was observed at 0.8 μg/ml of CN, and at 2.2 μg/ml with canine platelets [Trikha, M. et al., Journal of Cellular Biochem. 16F: 180 (1992)].
CN was reported to inhibit binding of human fibrosarcoma (HT-1080) and c-Ha-ras transfected rat embryo (4R) cells to fibronectin coated plates but not to matrigel coated plates. Inhibition of 4R cell binding to fibronectin in the presence of CN at 1 μg/ml and 5 μg/ml was 46% and 88%, respectively, and for HT1080 cells inhibition was 89% and 85%, respectively [Trikha, M. et al., Proceedings of the American Association for Cancer Research 33:34 (1992)].
Since it appears that CN can inhibit interactions between integrins and their receptors, the homodimeric disintegrin may prove useful in the management of diseases associated with these interactions. Consequently there exists a need for improved methods of making and using purified contortrostatin, substantially free other snake venom components.